What kind of agents should I be looking for?

I've finished my first novel and after polishing it up a bit creating a synopsis and query letters I decided it was time to go on the hunt for an agent. I live in new york so I'm fortunate to live in a city where a literary agency exist every five blocks. I recieved my first rejection letter, the woman was very kind and told me that although she saw potential in my novel, she didn't feel like she was an appropriate agent to represent it.

She then listed a list of genres she typically takes and although fantasy was one, my fantasy is admittedly more mature and intended for an audience of maybe age 17 and older. My question is how can you decide if an agent will feel comfortable with your kind of novel before you send them an query letter? In my case is there a way to be more certain even if they list your genre?

the only clue you can find beyond that is to check out the books she has successfully represented, to see if any like yours are among them... but even if there aren't, that doesn't necessarily mean she won't even consider other variations on the genre...